Optical element

ABSTRACT

A component has a substrate ( 1 ) made of a transparent material, for example glass. On this layer ( 1 ), there is a linear polarizer ( 2 ) on which there is a layer ( 3 ) of a photo-oriented polymer network (PPN)(=LPP) which is oriented in locally varying fashion via its surface which covers the substrate. The layer ( 3 ) is adjoined by an anisotropic layer ( 4 ) of cross-linked liquid-crystal monomers. This layer ( 4 ) then has a molecular arrangement whose orientation is defined by the underlying orientation layer ( 3 ). The layer ( 4 ) will have been photocross-linked by exposure to a suitable wavelength of light, with the result that the molecular orientation defined by the PPN layer ( 3 ) is fixed. The element denoted as a whole by 7, can then be used as an optical component which is protected against forgery, it being possible for the orientation pattern of the liquid-crystal layer or the optical information stored therein to be made visible by means of an external polarizer ( 5 ), for example.

[0001] The invention relates to an optical component containing anoptically anisotropic layer, which latter has at least two regions withdifferent molecular orientations. The anisotropic layer may, forexample, be a retarder layer formed by cross-linked liquid-crystalmonomers.

[0002] A particular use of the components according to the invention isin the field of protection against forgery and copying.

[0003] The demand for safeguarding banknotes, credit cards securities,identity cards and the like against forgery is increasing constantly onaccount of the high-quality copying techniques which are available.Furthermore, in low-wage countries, imitations of branded products andcopies of copyright-protected products, for example compact discs,computer software, electronics chips, etc. have been produced andexported worldwide. Because of the increasing number of forgeries, thereis therefore a great need for new elements which are safeguarded againstforgery and can be identified both visually and by machine.

[0004] In the field of copy-protecting banknotes, credit cards etc,there are already a considerable number of authentication elements.Depending on the value of the document to be protected, very simple orrelatively highly complex elements are employed. Some countries arecontent to provide banknotes with metal strips which come out black on aphotocopy. Although this prevents them from being copied, elements ofthis type are very easy to imitate. In contrast to this, there are alsomore complex authentication elements, for example holograms andcinegrams. Authentication elements of this type are based on thediffraction of light by gratings and need to be observed under differentviewing angles in order to verify their authenticity. These diffractedelements produce three-dimensional images, colour variations orkinematic effects which depend on the angle of observation and have tobe checked on the basis of predetermined criteria or rules. It is notpractically possible to use machines for reading information, forexample images or numbers, encoded using this technique. Furthermore,the information content of these elements is very limited, and only anoptical specialist will be capable of discriminating definitivelybetween forgeries and an original.

[0005] Lastly, one should not ignore the fact that diffractive opticaleffects have in the course of time also been used outside the field ofsecurity, in particular for consumer articles such as wrapping paper,toys and the like, and the production methods for such elements have inthe course of time become known to a large group of people and arecorrespondingly straightforward to imitate.

[0006] Further to the diffractive elements mentioned above, othercomponents are also known which are suitable for optimum copyprotection. These include optical components, as disclosed for exampleby EP-A 689,084 or EP-A 689,065, that is to say components with ananisotropic liquid-crystal layer, which latter has local structuring ofthe molecular orientation.

[0007] These components are based on a hybrid layer structure whichconsists of an orientation layer and a layer which is in contact with itand consists of liquid-crystal monomers cross-linked with one another.In this case, the orientation layer consists of a photo-orientablepolymer network (PPN)—synonymous with LPP used in other literaturewhich, in the oriented state, through a predetermined array, definesregions of alternating orientations. During the production of theliquid-crystal layer structure, the liquid-crystal monomers are zonallyoriented through interaction with the PPN layer. This orientation which,in particular, is characterized by a spatially dependent variation ofthe direction of the optical axis, is fixed by a subsequentcross-linking step, after which a cross-linked, optically structuredliquid crystal (LCP for liquid crystal polymer) with a preestablishedorientation pattern is formed. Under observation without additionalaids, both the orientation pattern itself and the information writteninto the liquid crystal before the liquid-crystal monomers arecross-linked, are at first invisible. The layers have a transparentappearance. If the substrate on which the layers are located transmitslight, then the LCP orientation pattern or the information which hasbeen written become visible if the optical element is placed between twopolarizers. If the birefringent liquid-crystal layer is located on areflecting layer, then the pattern, or the corresponding information,can be made visible using only a single polarizer which is held over theelement. LCP authentication elements make it possible to storeinformation, virtually without restriction, in the form of text, images,photographs and combinations thereof. In comparison with prior artauthentication elements, the LCP elements are distinguished in that theauthenticity of the security feature can be verified even by a laymansince it is not first necessary to learn how to recognise complicatedcolour changes or kinematic effects. Since LCP authentication elementsare very simple, reliable and quick to read, machine-readable as well asvisual information can be combined in the same authentication element.

[0008] As is likewise already known, the complexity of LCPauthentication elements can be increased further by inclining theoptical axis of the LCP layer relative-to the plane of the layer,uniformly or with local variation. This can be done in known fashion byproducing a PPN layer with a locally varying tilt angle on the surface.This further provides a tilt effect, that is to say the informationcontained in the birefringent layer is seen with positive or negativecontrast depending on the angle of observation. The object of theinvention is now to provide further possible layer structures of theabove-mentioned type for optical components, electro-optical devicesand, in particular, for copy protection elements.

[0009] According to the invention, this is achieved in that the physicalparameters and the configuration of the cross-linked liquid-crystallayer are varied and/or different layers, as well as a variety ofsubstrates, with differing respective optical properties are combined.Since the layers which are used are generally transparent, they can alsobe applied successfully to already known, permanently visibleauthentication elements, for example watermarks, holograms or cinegrams.The retarder pattern of the liquid-crystal layer can then be seen inaddition to the permanently visible authentication element onobservation using a linear polarizer.

[0010] When using the transmissive birefringent layers described in EP-A689,084, it is necessary to arrange one polarizer on each side of theelement in order to read or make visible the information which isstored. A quick check of identity cards and the like is in this casemade difficult by the involved positioning of the two polarizers aboveand below the authentication element. This disadvantage can be removedaccording to the invention by additionally integrating at least onepolarization layer in the layer structure. If there is, for example, apolarization layer below the birefringent layer, then one externalpolarization sheet, held over the element, is sufficient for making thestored optical information visible.

[0011] A polarization layer integrated in the authentication elementcan, according to EP-A 689,084, be designed as a dichroic LCP layer. Itis also possible to use a polarization sheet as a substrate for the PPNand LCP layers applied to it.

[0012] Where a reflector is present, which can be omitted according tothis invention, the polariser sheet may possibly be the polariser foringoing light and the analyser for outgoing light, which may not alwaysbe desirable.

[0013] A further disadvantage of the authentication elements describedin EP-A 689,084 is that, when arranging a polarizer below the substrate,the polarization state of the light on passing through the substrate canbe affected. If, for example, use is made of inexpensive polymersubstrates which, by virtue of the way in which they are produced, arethemselves birefringent then since the birefringence of these substratesis a random result of manufacture and varies from place to place, thebirefringence of the LCP layer may, in the extreme case, be cancelledout, with the result that the information of the authentication elementcan no longer be read. Furthermore, the use of strongly scatteringmaterials such as paper as a substrate is ruled out since polarizedlight would be immediately depolarized by these materials, so that thepolarization state of the light which passes through and is analyzedusing the second polarizer is unidentified and does not therefore carryany coded information.

[0014] However, if the integrated polarizer is, as proposed according tothe invention, located between the substrate and the LCP layer, then thesubstrate has no effect on the polarization state of the light onpassing through the LCP layer. As a result, on the one hand, it ispossible to use inexpensive polymer substrates which, by virtue of theway in which they are produced, are themselves birefringent, and on theother hand the substrate need not be transparent. In this case, evenscattering substrate materials can, for example paper and the like, arethus appropriate.

[0015] There are a variety of products, for example paintings,documents, photographs, compact discs, semiconductor chips, in which theauthentication element need not be visible since this would impair theoverall appearance of the product or would draw the attention of apotential product forger to the authentication element. For these cases,the invention proposes that orientable fluorescent dyes be incorporatedin a transmissive structured LCP layer.

[0016] There are yet further optical effects which can be used forliquid-crystal authentication elements. Examples include those producedby cholesteric filters. A known feature of these filters is that theyrefract, with circular polarization, a fraction of the visible lightspectrum in a wavelength range depending on physical parameters, whilethe unreflected light is transmitted (see: Schadt M., Fünfschilling J.,Jpn. J. Appl. Phys., 29 (1990) 1974). The effect of this is that thetransmitted light and the reflected light have different colours. Inorder for this to produce visual effects, it is necessary for thewavelength range of the selective reflection to lie in the visible lightrange. For applications in which the information is read by machine, itis of course possible for the refraction band to lie outside the visiblewavelength range.

[0017] Different types of optical components, which can likewise be usedas authentication elements in the field of copy protection, are based onthe combination of a linear polarizer with a cholesteric filter. Aconfiguration of this type makes it possible (as also further explainedbelow) to produce different colours, for which use is in particular madeof a second linear polarizer arranged on the opposite side of thecholesteric filter from the first polarizer.

[0018] Lastly, the tilt effect described at the start can also beproduced in a different way than is already known. It is thus possible,according to the invention, to produce authentication elements whosetilt effects are more pronounced and whose production is even simplerfrom a technical point of view. This is achieved, in particular, in thatat least one birefringent LCP layer of an element is constructed in sucha way that its effective birefringence depends on the angle ofobservation. In this case, the optical axis may lie in the plane of thelayer, i.e. it is not necessary to incur the extra cost of tilting theoptical axis out of the plane in a defined way.

[0019] According to the present invention, there is provided an opticalcomponent comprising at least two layers, characterized by a retarderand a polarizer, the retarder having at least two regions with differentoptical axes. Preferably the retarder comprises an anisotropic layercomprising cross-linked liquid-crystal monomers. The retarder may beplaced on an orientation layer and the orientation layer may be incontact with a polarizer. The orientation layer preferably comprises aphoto-oriented polymer network (PPN). The polariser may be placed on asubstrate. Optionally, a second polariser is arranged over theliquid-crystal layer and a further orientation layer and furtherliquid-crystal layer are arranged over this second polarizer, and thesecond liquid-crystal layer may also be structured. A further polarisermay be arranged over the second liquid-crystal layer, and a thirdorientation layer and a third liquid-crystal layer are arranged overthis further polarizer, and the third liquid-crystal layer may also bestructured. An element for protection against forgery and/or copying mayhave an optical component as set forth above and an external linear orcircular polarizer, the liquid-crystal layer encoding information whichcan be analyzed using the external polarizer. Such an element may becharacterised in that the at least two liquid-crystal layers each encodea partial information content which together form a total informationcontent. In this element, the liquid-crystal layer may be designed as aretarder and is preferably placed on a substrate characterized in thatthe substrate encodes a part of the total information content.Preferably, the external linear polarizer is structured, and both theliquid-crystal layer and the external polarizer each encode part of thetotal information content.

[0020] The optical component may be characterised by at least onecircular polarizer, or preferably by two circular polarizers arrangedone above the other, one of which rotates to the left and the other ofwhich rotates to the right. An element for protection against forgeryand/or copying may contain such an optical component and an externallinear or circular polarizer for analysing the encoded information.

[0021] The invention also provides an optical component comprising anoptically anisotropic layer which is formed by liquid-crystal molecules,characterised in that the optically anisotropic layer containsfluorescent molecules, and preferably has at least regions withdifferent optical axes. The invention extends to an element forprotection against forgery and/or copying including such an opticalcomponent.

[0022] The invention also provides an optical component comprising atleast two layers, characterized by a cholesteric layer and a linearpolarizer and preferably by an optically anisotropic layer, which mayhave regions with different optical axes. The optically anisotropiclayer may be formed of cross-linked liquid crystal molecules. Thecholesteric layer and the optically anisotropic layer are preferably onthe same side of the linear polariser, which may be in contact with thecholesteric layer. The linear polariser may be arranged on a substrate,the cholesteric layer being in contact with the linear polariser, and anorientation layer may be placed on the cholesteric layer, and anoptically anisotropic layer of cross-linked liquid-crystal monomers maybe placed on the orientation layer, the liquid crystal (opticallyanisotropic) layer forming regions with different molecularorientations. An element for protection against forgery and/or copyinghave such an optical component and an external linear polarizer foranalysing the information encoded in the liquid-crystal layer and/or inthe cholesteric layer.

[0023] The invention also provides an optical component, containing abirefringent liquid-crystal layer which has at least two regions withdifferent optical axes, characterized in that the optical delay of theliquid-crystal layer in the individual regions depends differently onthe angle of observation. This component may be designed in such a waythat the colour of the clement on observation through a polarizerdiffers locally, and may be biaxial; preferably the birefringent layeris biaxial. An element for protection against forgery and/or copying mayhave such an optical component. A further element, according to theinvention, for protection against forgery and/or copying comprises apolariser layer which has at least two regions with differentpolarisation directions.

[0024] A further such element is arranged on a substrate and comprisesan optically anisotropic layer which has at least two regions withdifferent optical axes, the substrate being a reflective polariser.

[0025] The invention also provides a device for protection againstforgery and/or copying, wherein an element of any of the types set forthabove and an analyser are arranged on the same substrate, such as acertificate or banknote.

[0026] Some of these may be considered as documents carrying invisibleproof of authenticity, often in polarised light form. Some suchdocuments, lacking a reflective layer, may be authenticable usingillumination from underneath (transmitted through the document to theviewer). Some such documents may advantageously lack an integratedpolariser.

Illustrative embodiments of the invention will now be described belowwith reference to the appended drawing. In a simplified schematicrepresentation,

[0027]FIG. 1 shows a layer structure of an optical component consistingof a polarizer, a PPN layer and an LCP layer, as well as the associatedanalyzer,

[0028]FIG. 2 shows the LCP structuring of the component in FIG. 1,

[0029]FIG. 3 shows a foldable document with an element of the typecharacterized in FIG. 1,

[0030]FIG. 4 shows a layer structure constructed in an alternative wayto the structure in FIG. 1, with an additional PPN layer and LCP layer,as well as an analyzer which is arranged after the layer structure inthe direction in which light travels,

[0031]FIG. 5 shows a layer structure constructed in an alternative wayto the structure in FIG. 1, with an additional PPN layer and LCP layer,as well as an analyzer which is arranged before the layer structure inthe direction in which light travels,

[0032]FIG. 6 shows a layer structure constructed in an alternative wayto the structure in FIG. 1, with two additional PPN and LCP layers, aswell as two external polarizers on opposite sides,

[0033]FIGS. 7a and 7 b show an LCP component which has a locallydifferent orientation and a cholesteric filter, as well as a polarizerarranged after the element in the direction in which light travels,

[0034]FIGS. 8a and 8 b show a layer structure which is of the type shownin FIG. 7, but with a polarizer arranged before the element in thedirection in which light travels,

[0035]FIGS. 9a and 9 b show a layer structure which is of the type shownin FIG. 7 but with an additional cholesteric filter,

[0036]FIGS. 10a and 10 b show a layer structure which is of the typeshown in FIG. 7, but in which the cholesteric filter and the polarizerare interchanged,

[0037]FIG. 11 shows a two-layer authentication element consisting of acholesteric filter and a first linear polarizer, as well as anassociated analyzer, and

[0038]FIG. 12 shows a layer structure as in FIG. 11, but with anadditional retarder layer.

[0039] The schematic section represented in FIG. 1 through a layerstructure of a first illustrative embodiment according to the inventionshows a substrate 1, made of a transparent material such as glass, forexample, or of a scattering material, such as paper, for example. On thesubstrate, there is a linear polarizer 2, on which there is a layer 3 ofa photo-oriented polymer network (PPN) whose orientation varies locally(e.g. imagewise) over its surface on the substrate. Examples ofmaterials which are suitable for this include cinnamic acid derivatives,as described for example in EP-A 525,478 or U.S. Pat. No. 5,389,698.They are oriented and at the same time cross-linked by selectiveexposure to linearly polarized UV light.

[0040] An anisotropic layer 4 of cross-linked liquid-crystal monomersadjoins the layer 3. This LCP layer 4 consists in this case of amolecular arrangement whose orientation is predetermined by theorientation of die underlying layer 2. Using light of a suitablewavelength, the LCP layer 4 is photo-cross-linked, by means of which themolecular orientation defined by the PPN layer 3 is fixed. Using anexternal polarizer 5, the orientation pattern or the stored opticalinformation (i.e. the image) can be made visible, for which purposelight passes from below in the direction of the arrow 6 through theelement denoted overall as 7, and the polarizer 5 (acting in this caseas an analyzer) is held over the element 7.

[0041]FIG. 2 shows the preferred mutual orientation of the optical axesof adjacent locally structured regions of the LCP layer 4. In order toproduce maximum contrast, the optical axes of adjacent regions areangled at 45□.

[0042]FIG. 3 shows a variant, still according to the invention forsimplifying the verification of such LCP security items. In this case,the second (external) polarizer 5 is mounted on a light-transmittingflexible substrate 8, such as a document or a banknote.

[0043] This is done in such a way that the polarizer 5 can be positionedover the element 7 mounted elsewhere on the same banknote, by folding orbending the banknote 8, so that the stored LCP image can be seen throughthe polarizer 5 on looking through. In this way, both polarizers neededfor recognizing the stored authentication element are present on thesame substrate, with the result that it is not necessary to haveexternal polarizers, and thus no further aids for analysing theinformation are necessary.

[0044] Of course, the second polarizer 5 may itself again form part of alayer structure which in turn bears an LCP layer. On the one hand, thereare then simultaneously two LCP layer structures on one substrate, withinformation content which can be made visible separately from oneanother as individual patterns, in each case using an externalpolarizer. On the other hand, the optical anisotropies of the two LCPlayers can also be combined with one another if the substrate is bentand then viewed through two polarizers. In this case, a third pattern isproduced which differs from the two individual patterns.

[0045] Complexity, surprise, and the optical quality and informationcontent can all be increased according to the invention by making thelayer structure of two information-carrying LCP layers sandwiching apolarization layer. Depending on whether a second, external polarizer isthen arranged above or below the layer structure, one or other of theinformation contents can be seen. The arrangement of the layers ofcorresponding elements are represented schematically in FIGS. 4 and 5.In this case, the two PPN and LCP layers, respectively together forminga pair, are denoted 11 a and 11 b or 12 a and 12 b, respectively, andthe polarizer layer arranged between the two pairs is denoted 13. Theexternal polarizer which acts as an analyzer is here denoted 14 a or 14b, and the direction of the light is denoted 15.

[0046] If, however, one external polarizer, notionally 14 a and 14 b(not shown), is arranged both above and below, then both informationcontents are seen at the same time. If one or both external polarizersare rotated through 90°, the information contents will be invertedindependently of one another, that is to say represented in negative.For example, an image could be stored in one of the two LCP layers andcorresponding textual information could be written in the other. Bychoosing the arrangement of the polarizer, it is then possible to makeonly the image or only the text visible, or both visible at the sametime.

[0047] Analogously with the examples described above with reference toFIGS. 1 to 5, the number of PPN and LCP layers can be increased further.In the case of an element 29 having a three-layer structure (FIG. 6),the layers 21 a/21 b, 22 a/22 b and 23 a/23 b are separated from oneanother by two crossed polarizer layers 24 and 25. In this layerstructure, the central LCP layer 22 b arranged between the twopolarizers 24 and 25 can be produced according to the method disclosedin EP-A 689,084. With light 26 incident at right angles to the plane ofthe layer, the information in the central layer 22 b is in this casepermanently visible, while the information in the upper or lower LCPlayers 23 b and 21 b, respectively, can as described above be madevisible by arranging an external polarizer 27 or 28 above or below theelement 29. If both the polarizer 27 and the polarizer 28 are appliedsimultaneously to each side of the element 29, then the information inall three LCP layers can be made visible at the same time. In this way,for example, a single image can be broken down and distributed betweenthe three LCP layers 21 b, 22 b and 23 b. Only by arranging one or twoexternal polarizers will the individual parts of the image be recombinedto form the original image.

[0048] The information in the central LCP layer may, however, also becoded through locally varying tilt angle, or through tilt effects of thetype which will be described below, that is to say, for example, throughspatially varying directions of the optical axis relative to the planeof the layer. The result of this, in the case of the layer systemconsisting of three LCP layers with polarization layers lying inbetween, is that the information in the central image cannot initiallybe seen so long as the layer is viewed at right angles. Only onobservation at an oblique angle does the information in the centrallayer become visible, because of the different birefringence of regionswith different tilt directions for the optical axis. By using one or twoexternal polarizers, the information contents of the lower and/or upperlayers are then visible at the same time as the information in thecentral layer.

[0049] The complexity can be increased by further LCP layers which arerespectively separated from the others by polarization layers. Theinformation in each of the LCP layers can thus be stored differently,for example through local variation of the direction of the optical axisin the plane as well as out of the plane. As a result, the informationcontent in the individual layers can be viewed independently of eachother according to the angle of observation and the arrangement ofexternal polarizers.

[0050] Linearly polarizing layers can also be produced using LCP layerswhich contain dichroic dye molecules. The dichroic molecules orient insuch layers according to the local orientation of the LCP molecules, sothat light is linearly polarized locally in the layer, that is to sayaccording to the orientation of the dichroic dye molecules. Bystructuring the doped LCP layer, it is thereby possible to producepolarization layers with locally differing polarization direction. Thebrightness and/or colour of the birefringent layer between twopolarizers depends on the direction of the optical axis of the retarderlayer, as well as on the transmission directions of the two polarizers,one(or both) of the polarizers needed to visualize the retarder patterncan themselves be structured and therefore carry information. Thepatterns in the retarders and polarizers can then be matched with oneanother. It is thus possible to put one part of the information in theLCP layer and another part in the polarization layer. The totalinformation content can then be read only by an individual who isprovided with the structured polarizer matching the retarder layer. Ifthere is a reflector under the structured retarder layer, then thesecond (optionally unstructured) polarizer underneath the retarder layeris no longer required for reading the information. However, just as partof the information content can be put in the analyzer, part of theinformation may already be present permanently on the substrate. In thisway, for example, a photograph can be broken down into a partpermanently visible on the substrate, and an initially invisible partwhich is put in the retarder layer and cannot be seen unless a polarizeris used. In the case of a LCP pattern on a reflector, a further variantcould be the reflector itself structured. On observation through apolarizer, the additional information which is stored in the structuredretarder layer is then seen inside the reflecting regions.

[0051] As already mentioned at the start, there a variety of products,for example paintings, documents, photographs, compact discs andsemiconductor chips, in which the authentication element is not intendedto be visible. Transmissive structured retarder layers would satisfythis condition, but in order to visualize the information which theycontain, a polarizer is placed before and after the retarder layer,which is possible only if the substrate does not alter the polarizationstate of the light. In contrast, in the case of reflective elementsbased on structured retarders, it is necessary for there to be areflector, which as a rule can always be seen, under the retarder layer.

[0052] For cases of this type, it is a further object to provide anauthentication element which although carrying retrievable information,cannot be seen under normal conditions. According to the invention, thisis achieved in that the orientable fluorescent dyes, which eitherfluoresce anisotropically or (and) absorb light anisotropically and haveabsorption bands in the UV range, are incorporated in a structured LCPlayer. If the fluorescent molecules are chosen suitably, than onexposure to polarized UV light, those molecules whose transition momentis parallel to the polarization direction of the exciting UV light, arepreferably excited. In an LCP layer in which the fluorescent moleculesare zonally perpendicular to one another in accordance with the LCPorientation, only those regions whose orientation is parallel to thepolarization direction of the UV light will consequently fluoresce, andthis makes it possible to see the information stored in the layer whichis invisible in the absence of UV excitation.

[0053] As an alternative, the doped LCP layer may also be excited withisotropic light. If the fluorescent molecules are chosen suitably, theyradiate the fluorescent light with a polarization, the direction of thepolarization being determined by the orientation of the molecules. Usinga polarizer, it is possible to discriminate between regions withdifferent polarization of fluorescent light, and this makes it possibleto see the information present in the layer.

[0054] FIGS. 7 to 10 show optical elements with at least one cholestericfilter which, as already mentioned at the start, can also be used forauthentication elements with cross-linked liquid-crystal molecules.

[0055] In a first illustrative embodiment (FIGS. 7a and 7 b) of thiscategory of elements, use is made of a structured LCP retarder layer 31whose optical delay or path difference is λ/4, and in which theinformation is encoded by means of regions whose optical axes areperpendicular to one another. If a cholesteric filter 33 whose selectivereflection wavelength λ_(R) lies in the visible light range is thenplaced under this structured retarder layer 31, or under its PPNorientation layer 32, then light passing through the cholesteric filterin the direction of the arrow 34 from below, will first be circularlypolarized in the region of the selective bands. On passing through thestructure retarder layer 31, the circularly polarized light will then beconverted into linearly polarized light because of the λ_(R)/4 opticaldelay. Since, as represented in FIG. 7b, the optical axes in thedifferently oriented regions are perpendicular to one another, thepolarization direction of the linearly polarized light after passingthrough the corresponding regions is also rotated through 90° relativeto one another. If a linear polarizer 35 having a transmission angleβ=45°, measured relative to the directions of the mutually perpendicularoptical axes of the retarder layer 31, is then held over thisarrangement, then coloured and colourless regions will be seen. When thepolarization 35 is rotated through 90°, the optical properties of theregions will be interchanged.

[0056] On the other hand, if the light is not put into the elementthrough the arrangement consisting of the filter 33, the PPN layer 32and the retarder layer 31, but is incident through the linear polarizerfrom above, as shown in FIG. 8, then the pattern which has been writtenwill be seen in complementary colours in the reflective light. In thisway, it is possible to produce authentication elements with highinformation content, in which the information appears as complementarycolours depending on whether the transmitted or reflected light isobserved.

[0057] Both the circular polarizer and the linear polarizer may formpart of the layer structure, in which case they are permanently present.They may, however, be arranged above or below the layer only when theinformation is read. A circular polarizer layer may, for example, beformed from a layer of chiral LCP material which is only a fewmicrometres thick.

[0058] The element represented in FIG. 9a has a similar design to theelement in FIG. 7, and has a structured retarder layer 41 with anoptical delay of about λ/4. In this case as well, the information isencoded by means of regions with mutually perpendicular optical axes, asshown in FIG. 9b. In this illustrative embodiment, however, oneleft-rotating and one right-rotating cholesteric filter 42 and 43,respectively, are arranged in series under the PPN layer 44 belonging tothe retarder layer 41. The maxima of the selected reflection bands ofthe two filters 42 and 43 lie in different wavelength ranges. If alinear polarizer 45 is again held over the structured retarder layer,then the regions with mutually perpendicular optical axes appear withdifferent colours. When the polarizer or the retarder layer is rotatedthrough 90□, the colours of the regions are interchanged.

[0059] A final further element in this category is shown by FIGS. 10aand 10 b. In this case as well, use is made of a structured λ/4 retarderlayer 51, in which the information is encoded by means of regions withmutually perpendicular optical axes. In contrast to the example in FIG.7, the linear polarizer 52 and the cholesteric filter 53 are in thiscase interchanged. Light incident form below in the direction of thearrow 54 will firstly undergo uniform linear polarization by the linearpolarizer 52 and, on passing through the structured retarder layer 51,will become left or right circularly polarized depending on thedirection of the optical axis. If a cholesteric filter 53 which acts asa circular polarizer is held above, then either left or right circularlypolarized light will be transmitted, depending on the sense of rotationof the circular polarizer, and light with the opposite sense of rotationwill be reflected. The pattern written in the retarder layer 51, encodedby different optical axis directions, then appears as a pattern ofbright coloured regions.

[0060] If, in this special case, the circular polarizer 53 is replacedby a second linear polarizer, then the pattern cannot be seen since thepolarization state of the light after passing through the regions of theretarder layer 51 is either right or left circularly polarized.

[0061] The fact that retarder regions whose optical axes are mutuallyperpendicular cannot be distinguished using linear polarizers, opens upthe possibility of writing different information contents in an LCPlayer, it being possible for these to be read independently of oneanother using different aids. To do this, for example, firstinformation, as described in the illustrative embodiment in FIG. 10, canbe encoded using regions with mutually perpendicular optical axes.Second information is then encoded using regions whose optical axes makean angle of 45° with the mutually perpendicular axes in the said firstregions. If, as described in the illustrative embodiment in FIG. 10, alinear polarizer is placed under the retarder layer and the layer isilluminated through it, then only the second information is seen whenusing a second linear polarizer which is held over the element formed bythe linear polarizer, PPN layer and LCP layer. In contrast, the firstinformation is seen with a normal observation angle, only if (as alreadyexplained) a circular polarizer instead of the linear polarizer is heldover the retarder layer, and in this case the second information canalso be seen with a reduced intensity. In an authentication element, itwould thus, for example, be possible to have a polarization layerpermanently integrated under the structured retarder layer, so that inorder to verify the authenticity of the element, it is sufficient tohold the linear polarizer and circular polarizer successively over thesaid element in order to see the different information contents.

[0062] Lastly, it will be pointed in this regard that, when at least onecholesteric filter is used, there is the further possibility, in orderto visualize a retarder structure, of not using any linear polarizers,but only using circular polarizers. The information is, for thispurpose, recorded by structuring the optical delay in the retarderlayer, it then being possible for the optical axis to have the samedirection throughout the plane of the layer. If a retarder layer of thistype is placed between two cholesteric filters whose selectivereflection bands overlap, then the information which is written will bevisible or readable.

[0063] As already mentioned at the start, there is a further possibilityof developing optical authentication elements which are essentiallyformed by a cholesteric filter and two linear polarizers.

[0064] This is because combining a linear polarizer with a cholestericfilter makes it possible to produce different colours if a second linearpolarizer, used as an analyzer, is arranged on the opposite side of thecholesteric filter from the first polarizer.

[0065] In the simplest case, an authentication element employing thiseffect would consist only of one cholesteric layer. In order to producean optical effect usable for authentication elements, it is thennecessary to have two linear polarizers which, as required, are to beheld over or under the cholesteric layer. In this simple case, thecholesteric layer may only be applied to a transparent substrate, forexample glass. However, if the authentication element is to be appliedto a diffuse depolarizing substrate, then the first polarizer may beintegrated permanently in the authentication element. An authenticationelement of this type is represented in FIG. 11. This element consists ofa cholesteric layer 61, a substrate 62 and a first polarizer 63,arranged between the substrate 62 and the layer 61. The second linearpolarizer, required for observing the stored information, is denoted 64and, when required, should be held over the said element.

[0066] The colour of the light passing in the direction of the arrow 65through the linear polarizer 63 in the cholesteric filter 61 is firstlydetermined by the wavelength of the selective reflection of thecholesteric filter 61. If the external linear polarizer 64 is then heldover the cholesteric layer, then the colour changes when the polarizer64 is rotated. If, for example, use is made of a cholesteric filter 61which reflects the colour green, then it firstly appears red-violet intransmission. Conversely, if the layer is observed through the secondpolarizer 64, then on rotation of this polarizer the colours yellow,green, red or blue are seen.

[0067] If a uniaxial optical delay layer with a path difference of forexample, λ/2, is then placed between the cholesteric filter 61 and thesecond polarizer 64, then for a constant position of the polarizer 64,the colours are changed by rotating the delay layer. Through suitablechoice of reflection wavelength and bandwidth for the cholestericfilter, and through suitable choice of the optical path difference andthe direction of the optical axis of the delay layer, it is in this waypossible to produce a broad palette of colours. Instead of between thecholesteric filter 61 and the polarizer 64, the delay layer may also belocated between the input polarizer 63 and the cholesteric filter 61.

[0068] So long as an unstructured delay layer is used, the coloureffects do not differ very greatly from those achieved using a singlecholesteric layer between two polarizers. However when use is made ofstructured retarder layers in which the optical axis zonally has adifferent alignment, it is possible to produce locally differentcolours. One embodiment of an authentication element designed in thisway is represented in FIG. 12. It consists of a first polarizer 72,placed on a substrate 71, a cholesteric layer 73 and a structured LCPretarder layer 74 with an associated PPN orientation layer 75. If thiselement is placed under an external polarizer 76 whose polarizationdirection is, for example, perpendicular to the polarization directionof the polarizer 72, then different colours are seen, the number ofdifferent colours depending on the structuring of the retarder layer 74and being determined by the number of differently aligned optical axes.In this way, information can be represented in colour. This impressiveoptical effect is further enhanced in that the respective colours changewhen the polarizer 76 is rotated. Furthermore, on account of thedependence of these selective reflection wavelengths on viewing angle,and on account of the optical path difference in the retarder layer, anauthentication element of this type has a pronounced colour dependenceon observation angle.

[0069] Further to structuring the direction of the optical axis, it isalso possible to structure the optical delay in the retarder layer. Itis thereby possible to optimize the appearance of colour using anadditional parameter.

[0070] Although the combination of cholesteric filter and optical delaylayer makes it possible to represent a large number of colours, it isnevertheless not possible with this arrangement to adjust the brightnessof the colours over the full range from dark to bright. This can,however, be achieved by structuring the cholesteric filter, for exampleby locally removing the cholesteric layer by photolithography, or byshifting the reflection wavelength of the cholesteric filter when it isbeing produced by locally varying the path length in the invisiblewavelength range. Since the cholesteric filter is not present, or isoptically isotropic, at points treated in this way, only the retarderlayer determines the optical behaviour at these points. In the case ofcrossed polarizers, it is for example possible for the optical axis tobe set parallel to one of the polarizers, as a result of which the lightat this point is blocked and therefore appears dark. By varying theratio of the areas of dark and coloured regions, it is thus possible tocontrol the brightness of the individual colours (mosaic picture).

[0071] As already mentioned above, the tilt effect described at thestart in birefringent layers can also be produced in a different waythan is already known, by means of which it is possible to makeauthentication elements whose tilt effect is more pronounced and whoseproduction is even easier to carry out.

[0072] According to the invention, this is achieved in that at least onebirefringent layer of the layer structure is constructed in such a waythat its effective optical delay depends on the angle of observation inthis case, the optical axis may lie in the plane of the layer, i.e.there is no need to pay the extra cost of tilting the optical axis outof the plane in defined fashion. The optical delay is equal to theproduct of the layer thickness and the optical anisotropy of thematerial, so that the optical delay for a given material can be adjustedby means of the layer thickness. Depending on the value of the opticaldelay, the layer appears with different colours or grey on observationusing crossed polarisers. If the effect of optical delay is thendependent on the angle of observation, then the grey value or the colourchanges correspondingly with the angle of observation. For example, witha material having positive, uniaxial optical anisotropy, the opticaldelay can be adjusted in such a way that the layer appears violet whenobserved vertically. If, however, the layer is viewed obliquely, in sucha way that the viewing angle and the optical axis form a plane, then thecolour changes from violet to yellow. If, however, one looks obliquelyfrom a direction which is perpendicular to the optical axis, then thecolour changes from violet to blue. With a corresponding position of theoptical axis, it is thus possible to achieve the effect that, when thelayer is tilted downward or upward, the colour changes from violet toyellow, while it changes to blue when the layer is tilted to the rightor to the left.

[0073] This angular dependence of the optical delay can be employed toproduce structured LCP authentication elements with information writtenin them, this information having an angle-dependent appearance. If, forexample, an LCP layer is structured as described in EP-A-689,084, insuch a way that the optical axis of different regions are, in accordancewith the information to be represented, either parallel or perpendicularto a reference axis lying in the plane of the layer, then theinformation cannot at first be seen under vertical observation withcrossed polarizers. Only when the layer is observed obliquely does itbecome possible to see the pattern which has been written, since theangle of observation is then different for regions whose optical axesare perpendicular to one another. If the layer thickness is then againadjusted in such a way that the optical delay appears violet under thevertical observation, then the colour changes from violet to blue ontilting about the reference axis in those regions where the optical axisis parallel to the reference axis, while the colour in the other regionssimultaneously changes from violet to yellow. If the layer is tiltedupward or downward, then the information thus appears yellow on a bluebackground or blue on a yellow background if the layer is tilted to theleft or to the right. Of course, by means of the layer thickness it islikewise possible to set other colours, grey values or combinations ofcolours with grey values. When grey values are used, a black and whiteeffect is obtained instead of the colour effect.

[0074] In order to produce birefringent layers whose apparent imagechanges with the viewing angle, both uniaxially and biaxiallybirefringent materials are suitable. However, the dependence on viewingangle can be enhanced further by using optically biaxial materials. If,for example, the refractive index perpendicular to the plane of thelayer is less than the refractive index in the plane of the layer, thenthe optical delay and therefore the tilt effect under obliqueobservation change much more than in the case of a uniaxial material.

[0075] Instead of using a biaxial material, the strong dependence ofviewing angle can also be achieved by a layer structure made of two ormore uniaxial layers, the optical axis in one layer being, for example,parallel or oblique with respect to the plane of the layer, while in asecond layer it is perpendicular to the plane of the layer. Throughsuitable choice of the ratio between the layer thicknesses, the tilteffect can be made more intense or weaker. If, furthermore, the layer inwhich the optical axis is parallel or oblique with respect to the planeof the layer is also structured, that is to say the projection of theoptical axis onto the plane of the layer points zonally in differentazimuthal directions, then under crossed polarizers with obliqueobservation, a pattern is seen whose colours or grey values change withgreat effect when the observation angle is altered only slightly.

[0076] In a further illustrative embodiment, the strong dependence onviewing angle can also be achieved by a layer structure which containsan unstructured optically biaxial layer as well as a structuredbirefringent layer of optically uniaxial material. This can, forexample, be brought about very simply by applying the structuredbirefringent layer directly onto an optically biaxial sheet.

[0077] Authentication elements having a dependence on viewing angle canalso be made by using a substrate which can polarize incident light as afunction of angle. This is, for example, the case with non-metallicsmooth surfaces, for example glass or plastic. Obliquely incident lightwhich is reflected from the surface of such materials is at least partlypolarized. Under a particular angle of incidence (the Brewster angle),which depends on the respective material, the reflected light is in factcompletely linearly polarized. If use is made of such a material withangle-dependent polarizing effect as a substrate for structured retarderlayers, then obliquely incident light which is reflected from thesurface of the substrate will be polarized before it passes againthrough the retarder layer. The polarization state is then changed as afunction of the local direction of the optical axis, so that a patterncan be seen in a correspondingly structured retarder layer if a layer ofthis type is viewed obliquely through a polarizer. The optimum contrastis achieved if the layer is viewed at the Brewster angle. The patterndisappears completely when the angle of observation is normal.

[0078] Instead of birefringent layers, it is also possible to producetilt effects by using layers which anisotropically absorb light. Layersof this type can, for example, be made with LCP layers in which dichroicdyes are incorporated. Since the dichroic dyes are oriented with the LCPmolecules, the dichroic dyes can likewise be given a zonally differentorientation though structured orientation of the LCP molecules. Onpassing through the layer, originally isotropic light then becomeslinearly polarized, the polarization direction being locally differentand determined by the local orientation of the LCP or dichroicmolecules. Depending on the dye which is used, it is possible topolarize light within the visible range or only within a singlewavelength range, so that the layers appear either grey or coloured. Thepattern which is written can be seen if the layer is observed through alinear polarizer.

[0079] LCP layers which contain dichroic dyes exhibit absorption whichdepends on the viewing angle. If a uniaxially oriented LCP layer whichis doped with dichroic dyes is tilted about the orientation direction ofthe LCP or dye molecules, then because of the increase in the opticalpath with increasing tilt angle, the layer appears darker than with anormal angle of observation. However, if the layer is tilted about anaxis lying perpendicular to the LCP orientation direction in the planeof the layer, then the layer appears brighter since the absorption axisof the dye molecules is in this case oblique with respect to theincidence direction of the light, which has the result that a smallerproportion of the light is absorbed. In order to see these variations inbrightness due to tilting, it is not absolutely necessary to observe thelayer through a polarizer. If, for example, an LCP layer is thenstructured in such a way that, in different regions, the LCP moleculeare parallel or perpendicular to one another, then when the layer istilted about one of these two preferential directions, those regionswith the LCP orientation parallel to the tilt axis appear darker, whilethe others appear brighter. Conversely, if the layer is tilted about theother preferential axis, then the brightness of the regions isinterchanged. It is possible to see this effect as well without using anadditional polarizer, and it is therefore particularly suitable forapplications where the intention is to check an authentication elementwithout an additional aid.

[0080] The production of a PPN and LCP layer which can be used accordingto the invention, as well as the production of an authentication elementwith a tilt effect, will be explained in more detail below.

[0081] 1. Production of a PPN layer

[0082] Suitable PPN materials include, for example, cinnamic acidderivatives.

[0083] For the investigations fundamental to the present invention, aPPN material with high glass point (T_(g)=133° C.) was chosen:

[0084] Polymer:

[0085] A glass plate was spin-coated with a 5 percent strength solutionof the PPN material in NMP for one minute at 2000 rpm. The layer wasthen dried for one hour at 130□C on a heating bench and for a furtherhour in a vacuum. The layer was then exposed to linearly polarizedlight, 200 W Hg high-pressure lamp for 5 minutes at room temperature.The layer was then used as an orientation layer for liquid crystals.

[0086] 2. Mixture of cross-linkable LC monomers for the LCP layer.

[0087] In the examples, the following diacrylate components were used ascross-linkable LC monomers:

[0088] Using these components, a supercoolable nematic mixture M_(LCP)with particularly low melting point (TM≈35° C.) was developed, making itpossible to prepare the LCP layer at room temperature.

[0089] The diacrylate monomers were present with the followingcomposition in the mixture: Mon1 80% Mon2 15% Mon3  5%

[0090] In addition a further 2% of the Ciba-Geigy photoinitiatorIRGACURE 369 were added to the mixture.

[0091] The mixture M_(LCP) was then dissolved in anisol. By means of theM_(LCP) concentration in anisol, it was possible to adjust the LCP layerthickness over a wide range.

[0092] For photoinitiated cross-linking of the LC monomers, the layerswere exposed to isotropic light from a 150 W xenon lamp for about 30minutes in an inert atmosphere.

[0093] 3. Authentication element with tilt effect

[0094] The two halves of a PPN-coated glass plate were exposed topolarized UV light, the polarization direction of the light whenilluminating the second half being rotated through 90° relative to thefirst exposure. In each case, the other half was covered during theexposure. This gave two regions with planar, mutually perpendicularorientation directions.

[0095] A 5 percent strength solution of M_(LCP) in anisol was produced.The solution was spun onto the PPN layer that had been exposed in thedifferent ways. Spin parameters: 2 minutes at 1000 rpm. In order tooptimize the orientation of the LC monomers, a coated substrate was thenheated to just above the clearing point (T_(c)=67° C.). The layer wasthen cooled at a rate of 0.1° C./min to three degrees below the clearingpoint.

[0096] After the LC monomers had cross-linked, the thickness of the LCPlayer which was obtained was about 80 nm.

[0097] If this layer is arranged between crossed polarizers in such away that the orientation directions of the LCP layer form an angle of45° with the transmission directions of the polarizers, then the LCPlayer appears uniformly grey. If, however, the layer is observedobliquely, with the viewing direction and the orientation direction ofthe left-hand half of the plate forming a plane, then the left-hand halfof the plate appears darker while the right-hand half of the plateappears lighter.

[0098] To conclude, it should be pointed out that the optical effectsdescribed above, as well as the corresponding layer structures andmaterial compositions, represent no more than a choice from a pluralityof embodiments according to the invention, and may in particular becombined in a wide variety of ways in order to develop authenticatingelements.

[0099] Thus, it is of course possible for any-other kind of birefringentlayer using which it is possible to produce an optical effect that canbe employed, for example for authentication elements, to be put into theoptical component instead of an LCP layer.

[0100] It is furthermore possible for the examples described above,instead of a PPN orientation layer, to use a different orientation layerwhich, according to the desired optical property and resolution, has thesame or similar properties to a PPN layer. It is also conceivable toproduce the orientation required for a retarder layer using acorrespondingly structured substrate. A structured substrate of thistype can, for example, be produced by embossing, etching and scratching.

[0101] Lastly, it should be pointed out that the multilayer structuresaccording to the invention can be used not only as elements forsafeguarding against forgery and copying, but for example can also beused to produce electro-optical liquid-crystal cells in which the LCPlayer fulfils various optical and orienting functions.

1-31. (Cancelled).
 32. An optical component comprising at least twolayers, one layer being a structured retarder and the other layer beinga polarizer, wherein the polarizer is a circular polarizer.
 33. Anoptical component according to claim 32, wherein the retarder isstructured by a structure of the optical delay.
 34. An optical componentaccording to claim 32, wherein the retarder is structured by having atleast two regions with different optical axis.
 35. An optical componentaccording to claim 32, 33 or 34, wherein the retarder comprises ananisotropie layer comprising cross-linked liquid crystal monomers. 36.An optical component according to claim 32, wherein two circularpolarizers are arranged one above the other, one of which rotates to theleft and the other of which rotates to the right.
 37. An opticalcomponent according to claim 32, wherein for the circular polarizer acholesteric layer is used.
 38. An optical component according to claim36, wherein for the circular polarizers cholesteric layers are used. 39.An optical component according to claim 38, wherein the two cholestericlayers, one of which rotates to the left and the other of which rotatesto the right, have reflection bands with maxima which lie in differentwavelength ranges.
 40. An optical component according to claim 37,further comprising a linear polarizer.
 41. An optical componentaccording to claim 38, further comprising a linear polarizer.
 42. Anoptical component according to claim 40, characterized in that thecholesteric layer and the structured retarder are on the same side ofthe linear polarizer.
 43. An optical component according to claim 41,characterized in that the cholesteric layer and the structured retarderare on the same side of the linear polarizer.
 44. An optical componentaccording to claim 40, wherein the linear polarizer is in contact withthe cholesteric layer.
 45. An optical component according to claim 40,wherein the linear polarizer is in contact with the structured retarder.46. An optical component according to claim 40, wherein the linearpolarizer is arranged on a substrate, the cholesteric layer is incontact with the linear polarizer, and an orientation layer is placed onthe cholesteric layer, and further wherein an optically anisotropiclayer of cross-linked liquid crystal monomers, which forms regions withdifferent molecular orientations, is placed on the orientation layer.47. An element for protection against forgery or copying, including anoptical component according to claim 32 and an external linear orcircular polarizer for analyzing encoded information.
 48. A device forprotection against forgery or copying, characterized in that an elementaccording to claim 47 and a linear or circular polarizer are arranged onthe same substrate.
 49. An optical component comprising an opticallyanisotropic layer which is formed by liquid-crystal molecules, whereinthe optically anisotropic layer contains fluorescent molecules.
 50. Anoptical component according to claim 49, wherein the opticallyanisotropic layer has at least two regions with different optical axes.51. An element for protection against forgery or copying comprising anoptical component comprising an optically anisotropic layer which isformed by liquid-crystal molecules, wherein the optically anisotropiclayer contains fluorescent molecules.
 52. An optical component,containing a birefringent liquid-crystal layer which has at least tworegions with different optical axes, wherein an optical delay of theliquid-crystal layer in the individual regions depends differently on anangle of observation.
 53. An optical component according to claim 52,wherein a color of the element on observation through a polarizerdiffers locally.
 54. An optical component according to claim 52,characterized in that it is biaxial.
 55. An optical component accordingto claim 54, wherein the bire-fringent liquid-crystal layer is biaxial.56. An element for protection against forgery or copying comprising anoptical component comprising a birefringent liquid-crystal layer whichhas at least two regions with different optical axes, wherein an opticaldelay of the liquid-crystal layer in the individual regions dependsdifferently on an angle of observation.
 57. An element for protectionagainst forgery or copying, being arranged on a substrate and comprisingan optical anisotropic layer which has at least two regions withdifferent optical axes, wherein the substrate is a reflective polarizer.58. A device for protection against forgery or copying comprising anelement and an analyzer, wherein the analyzer and the element arearranged on a single substrate.